This invention relates to the electrical measurement art, and more particularly to a new and improved non-contacting electrostatic voltage follower.
A d.c. electrostatic voltage follower is a device used to measure the d.c. voltage level of conducting or dielectric surfaces in a non-contacting manner. An important application of this device is in the measurement of the voltage levels of photoconductor surfaces used in electrophotographic processes, i.e. xerography. In these processes latent images are stored as electrical charges on photoconductive surface to produce different voltage levels corresponding to the optical image exposed on the surface. To read these voltage levels without disturbing the small charges which produced them, non-contacting electrostatic voltage followers are employed. Using an electrostatic voltage follower, the sensing probe, which is coupled to the photoconductor surface under test, is driven to the same voltage as the measured voltage level of the surface. This voltage following technique assures no disruption of the surface charge which would result in voltage level errors, due to capacitive loading of the surface by the probe, or which would cause damage to the surface due to electrical discharge or arcing between probe and surface, particularly where the high surface voltage levels of 1,000 volts to 2,000 volts are measured in electrophotography. The assurance of no capacitive error or discharge can only be maintained if a physically closely coupled sensing probe can follow all variations in surface voltage levels to maintain a zero voltage difference between probe and surface at all times.
Recently, due to technological advances in electrophotography, high speed copying and laser printing machines, both of which use electrophotography processes, have emerged. In these high speed processes, the photoconductive surface moves at a high speed and presents very fast variations of voltage levels to the electrostatic voltage follower probe coupled to it. It is necessary, therefore, to provide increased bandwidth or speed of response of the electrostatic voltage follower over currently available devices to provide accurate surface voltage information as well as to insure no capacitive errors and surface arcing.
In another application, where different chemical composition and structure of photoconductive surface materials are evaluated for their electrical and optical characteristics, a d.c. electrostatic voltage follower is employed to monitor surface voltage performance. In a typical evaluation test, where the light decay characteristic of a material is to be evaluated, the material is charged in the dark to an initial voltage level. The surface is then radiated with light of various intensities and wavelengths while the surface voltage level is monitored to determine the rate of speed at which the surface discharges, i.e. rate at which surface voltage changes, as a function of the intensity, wavelength, and other variables.
Recently, with the development of amorphous silicon and other improved light decay speed photoconductive surface materials, non-contacting electrostatic voltage followers having increased bandwith or speed of response are required to accurately indicate the light decay speed of the material. Heretofore, the electrostatic surface voltage measurement devices used for these surface evaluation measurements have been either of the wire loop type or of the transparent dual detector electrode type. The disadvantages of the use of wire loop detectors are numerous. They cannot detect the d.c. level of the surface and therefore are unstable and require repeated resetting. In addition, they do not couple to the same area on the surface which is being radiated and therefore introduce measurement errors. The transparent dual detector electrode types offer good bandwidth and d.c. stability, but they are large in size and do not transmit radiation to the surface uniformly because of the required gap between the detector electrodes. This gap allows the electrodes to operate independently, i.e. 180.degree. out of mechanical phase, but it offers different spectral transmission characteristics through the gap areas than through the transparent electrode areas, thus generating transmission and reflection errors.
The response speed of all current art d.c. electrostatic voltage followers which employ a single detector electrode is relatively slow as compared to the speed of electrostatic data to be collected on modern xerographic machines and materials. In the current art d.c. follower, the capacitance or electrostatic field between the detector electrode in the sensing probe and the surface to be measured is modulated at a rate or frequency which is high as possible consistent with the available driving and handling power of the mechanical modulator as well as satisfying the requisite of producing enough modulation to reduce system noise and d.c. errors to acceptable values. In general, as the frequency of modulation increases the efficiency of modulation decreases to cause increased error and noise, while the reliability of the mechanical modulator decreases due to increased mechanical stress and strain. For these reasons, current art d.c. electrostatic followers employ capacitance or electrostatic field modulators which are limited to approximately one to two kilohertz in operating frequency.
The one to two kilohertz modulation frequency limit imposes a limit on the useful bandwidth obtainable from the d.c. electrostatic voltage follower or from any similar system which employs a feedback loop composed of a modulator, demodulator, and d.c. error integrating amplifier. The bandwith is limited because the d.c. error signals from the demodulator, i.e. signals which indicate that the voltage follower is not following and holding the voltage difference between the probe and detector electrode and test surface at zero, cannot be generated any faster, i.e. at a frequency higher than the modulation frequency. This conventionally known limit known as the Nyquist Sampling Limit imposes a system bandwidth limit which is equal to one-half the frequency of modulation. Thus, current art d.c. electrostatic voltage followers using one to two kilohertz modulation frequency cannot have a bandwidth which exceeds 500 hertz to one kilohertz.